[coreboot] add Kconfig options in addition/in place of CMOS option

[Josef Kellermann]

Attached patch adds sane compile-time defaults via Kconfig options for 
CMOS options
  'iommu, ECC_memory, max_mem_clock, hw_scrubber, interleave_chip_selects'.
Avoid build error if any of these CMOS options are not defined.
Checks the return value of 'get_option(..)', if not done yet, and
if CMOS option not exist grab a Kconfig option otherwise a hardcoded
value is used.

Signed-off-by: Josef Kellermann